Thanks BadMario those are essential points (& in no way a hijack, these are all useful things to note in regards to this topic).
So I built a test. I made a copy of the Beat Em Up project I've well underway (which contains a fair few sprites already but only weighs in at around 25MB on disk),then I loaded it with a 18 individual sprites at 200px² each containing a whopping 160 individual animations at 35frames each. That's a ludicrous 100,800 image files. All pre-loaded in the first layout (I've not used the MM-Preloader plugin yet).
So on disk this is a huge 6½GB. Construct takes about 2 minutes to save/autosave the project each time, & on top of that all browsers I've tried just crap themselves & crash when I try to run a preview.
However when I export it, after brute compression, the NW.EXE for windows only weighs in at 94MB. The game loads almost instantly & runs fine. I even put in a test key to spawn different character sprites again & again. I had like 30 of these massive bastards on the screen all playing a looping animation & the game seemed to run with no problems.
I figured my PC was just powerful enough to chew it up & spit it out, so I tried it on my crusty 10 year old iMac. Even though the OSX export was twice the size at 196MB, Steve Jobs old paperweight still loaded the game almost instantly & run it with no problems, even with a butt-load of sprites getting spawned onto the screen.
Finally I tried it on this crappy old tablet I found. I have bricks in my back yard that have more processing power than this thing. I ran the 32bit windows EXE on it & even though it was a little slow it still run ok, if not great.
So the real problem here wouldn't be exporting & distributing the game it would actually just be working on it, as Construct will not preview a project of this size. I guess if I just build it gradually, piece by piece, this isn't going to be a huge problem? Any ideas for improving the creation process are welcome.